1. Field of the Invention
The field of the invention is that of surface-acoustic-wave filters with highly frequency selective performance characteristics, namely relatively small bandwidths with high rejection qualities. The term  less than  less than surface wave  greater than  greater than  is understood here to mean not only Rayleigh waves but also any type of wave that can interact with interdigitated electrodes on the surface of a crystal or at the interface between a crystal and one or more layers of any material. The waves known as Pseudo SAW or Leaky SAW waves, the waves known as Surface Transverse Waves or SSBW (Surface Skimming Bulk Waves) may thus be considered to be surface waves and the invention can be applied to this type of wave as well as to any type of wave that meets the above conditions.
2. Discussion of the Background
In general, to obtain performance characteristics of this kind, filters are used with resonators conventionally formed by transducers contained in a cavity formed by two periodic arrays as shown in FIG. 1. More specifically, these are multipole filters obtained by coupling several resonators. It is sought indeed to couple the greatest number of resonators since the number of resonators coupled to each other generally determines the shape factor of the filter, namely the ratio between the rejection band of the filter and its passband. The increase in the number of resonators makes it possible to approach a shape factor of 1.
At present, there are known ways of making a surface-acoustic-wave filter configuration with two poles comprising a first pair of resonators IDT1 with a center frequency f1, a second pair of resonators IDT2 with a center frequency f2, the set of two pairs being mounted in an  less than  less than electrical equivalent  greater than  greater than  bridge structure as shown in FIG. 2. This type of configuration is described in the U.S. Pat. No. 5,508,667.
The value of this structure lies in the fact that the product of the static capacitances of the first pair is the same as the product of the static capacitances of the second pair. This balancing makes it possible to ensure rejection by the filter far from its center frequency. Indeed, far from this frequency, the resonators are electrically equivalent to their static capacitance and the balancing makes it possible to prevent any signal from passing through.
To further increase the performance characteristics of the resonator filters, it is planned to set up a series cascade of several xe2x80x9celectrical equivalentxe2x80x9d bridge structures. Most frequently, identical bridges are cascade-connected with one another. The drawback of a structure of this kind lies in the fact that this cascade, in order to be efficient, requires the addition of a supplementary electrical element (an inductor or a capacitor). Furthermore, the use of several cascade-connected xe2x80x9cbridgesxe2x80x9d is an obstacle as regards the mounting inside the pack of the filter.
This is why the invention proposes a filter structure that makes it possible to do away with the need for the coupling inductance in which a single equivalent bridge structure may comprise a large number of poles, through a parallel connection of different resonators in each arm of the bridge.
More specifically, an object of the invention is a surface-acoustic-wave filter with N poles, N being a number greater than or equal to 3 and comprising a set of resonators, characterized in that:
the resonators are electrically coupled to form a four-arm electrical bridge;
two arms comprising two identical sub-assemblies E1 and E3 of N1 resonators each parallel-connected;
two arms comprising two identical sub-assemblies E2 and E4 of N2 resonators each parallel-connected;
with N1+N2=N;
the product of the total static capacitance of the sub-assembly E1 multiplied by the total static capacitance of the sub-assembly E3 bring substantially equal to the product of the total static capacitance of the sub-assembly E2 multiplied by the total static capacitance of the sub-assembly E4, so as to balance the electrical bridge.
According to a first variant of the invention, if N is an even number, N1=N2=N )/2.
According to a second variant of the invention, if N is an odd number, N1=(Nxe2x88x921)/2 et N2=(N+1)/2.
According to the filter configuration proposed in the invention, the parallel connection of the resonators requires connection wires or tracks on the substrate to make the bridges. To overcome this obstacle, a preferred mode of the invention consists not in making the parallel connection of the resonators physically but rather of the use, in the different arms of the bridge, of surface wave devices behaving like several parallel-connected resonators.
This is why, in at least one of the arms of the electrical bridge, the surface-acoustic-wave filter may comprise a single surface wave device having an admittance equivalent to the parallel connection of a sub-assembly of resonators.
In particular, the filter according to the invention may be characterized in that at least one arm comprises a structure equivalent to several parallel-connected resonators, said structure comprising two networks of interdigitated electrodes constituting the transduction part of the resonator, said networks being connected to two buses with different polarities in comprising m acoustic channels inserted between the two buses, the ith channel possessing a pitch pi of electrodes on a length of electrodes wi and 1xe2x89xa6ixe2x89xa6m.
According to one variant of the invention, the ith acoustic channel may comprise two reflective arrays on each side of the transduction part.
According to one variant of the invention, the surface-acoustic-wave filter may comprise, in at least one of the arms of the electrical bridge, a structure equivalent to at least two parallel-connected resonators, said structure comprising two interdigitated electrode arrays, said arrays being connected to a first bus and a second bus with different polarities so as to define a transducer having a central axis Z parallel to the electrodes, said arrays comprising a part of their electrodes positioned symmetrically with respect to the central axis, the transducer also comprising electrodes positioned symmetrically with respect to the central axis and connected to buses with different polarities so as to excite symmetrical longitudinal modes and antisymmetrical longitudinal modes. The arrays of electrodes may or may not be inserted between reflector arrays.
According to another variant of the invention, the surface acoustic filter comprises, in at least one of the arms of the electrical bridge, a DART type resonator with transduction cells interposed between reflection cells.
Advantageously, the DART type resonator may comprise resonant cavities.
When the number of poles is an even number, the distance between the transduction center of a transduction cell and the reflection center of the reflection cell adjacent to said transduction cell may preferably be in the range of (3xc2x1d)xcex/8+kxcex/2, with xcex being the wavelength corresponding to the center frequency of the filter, d being smaller than 1 and k being an integer.